The invention relates to an endoscope system for observing human tissue that is illuminated with light having a predetermined wavelength. More specifically, the human tissue is illuminated with light having a predetermined wavelength, resulting in the tissue fluorescing at another predetermined wavelength. The fluorescence of the tissue is detected and processed by the endoscope system.
It is known that when human tissue is illuminated by light which has a wavelength of between 400 nm (nanometers) and 480 nm (hereinafter referred to as excitation light), the tissue will fluoresce (glow), thereby producing light having an approximate wavelength between 520 nm and 600 nm. Cancerous human tissue, however, does not fluoresce even if it is illuminated with the excitation light. Therefore, cancer at an early stage, which may not be detected during a normal endoscope observation, can be detected with an endoscope system which illuminates the tissue with the excitation light (i.e., a fluorescence diagnosis endoscope system).
A conventional fluorescence diagnosis endoscope system has an excitation light filter between a light source and a light path of an endoscope, and a fluorescent light filter between an objective optical system and an image receiving element provided at the insertion side of the endoscope. The excitation light filter allows only the excitation light to pass through, and the fluorescent light filter allows only the fluorescent light to pass through. An example of such an endoscope system is disclosed in a Japanese Patent Provisional Publication HEI 4-150845.
Since the intensity of the light produced by the fluorescence of the illuminated tissue is low, it is sometimes difficult to obtain a fluorescent image of the tissue that is sufficiently bright. Therefore, it is difficult to perform the diagnosis accurately.
Further, due to the construction of the conventional fluorescence diagnosis endoscope system described above, only the light produced by the fluorescence of the tissue enters the image receiving element of the endoscope. Thus, the organs or the tissue cannot be observed when the tissue is illuminated with normal light by the same endoscope. In order to examine the organs or tissue using normal light, the endoscope for the fluorescence diagnosis is removed and another endoscope for the normal observation is inserted. This is both time consuming and disruptive during an examination of a person.